Unbound and lonely

Astronomy Free-floating planets outside the solar system are far away from their parent stars, but are not stone cold and have water in the form of liquid since they produce substantial heat. CFBDSIR 2149 is the first isolated planet of its kind to be discovered. C Sivaram explores the possibility of life on such planets.

The number of exoplanets that have been discovered now runs into several hundreds. However, for the first time, astronomers have reported the discovery of a young, isolated planet not orbiting any parent star and not bound by gravity. It is an ‘orphan’ planet freely floating in space not tied to any parent star which can illuminate and heat it.

The lonely planet, CFBDSIR 2149, is about seven times the mass of Jupiter, and apparently has its own heat source which keeps its surface temperature around 300 degrees Celsius. It is the first isolated planet of its kind ever to be discovered, like a single needle lurking amongst thousands of haystacks!

It has been suggested that such isolated planets have existed earlier and there could be many of them, other than exoplanets which are bound to the gravity of their parent star. Indeed there could be literally thousands of such objects, not bound gravitationally to any star, but floating about in interstellar space far from any of the stellar systems which could have ambient conditions suitable for exo-biological life.

It is difficult to detect them because of their low luminosity. However, some of the larger ones could still shine because of their internal energy sources. We should remember that large planets like Jupiter and Saturn in our own solar system emit at lease three times more radiant energy than what they receive from the sun!

Radioactive release of energy by the decay of unstable nuclear isotopes of various elements in their rocky cores is a substantial energy source. Indeed the earth emits more than 40 trillion watts (tera watts) from its surface owing to the radioactive decay of elements like potassium-40, apart from heavier isotopes of uranium, thorium, etc.

This is of course only a very small fraction of the radiant energy which the earth receives from the sun. But, for planets like Jupiter, this internal heat would be several pentillion (million trillion) watts. So, even if Jupiter, or for that matter the earth, were freely floating in space far away from the sun, they would not be stone cold, but radiating substantial heat. That is why the interior of the earth is quite warm or even hot, with all the molten rock in the form of lava spewing out of volcanic vents.

Sunlight does not penetrate deep, but the core of the earth is expected to have a temperature of several thousand degrees and is partially molten. This radioactive release of heat would have been more in the past. Four billion years ago, it would have been many times more. So the younger and larger the planet, the greater would be its internal release of heat and the warmer its surface.

Brown-dwarfs

On earth, life is said to have been found in volcanic vents and the floors of oceans, but this has hardly any connection with sunlight. This implies that many of these freely floating planets should be discounted as candidate objects of exobiological interest.

They could have their own internal energy sources, especially if they are very young massive Jupiter-like objects with liquids like water in their interior which would enable them to maintain temperate-ambient conditions conducive to living systems!Exoplanets come in many sizes. We have a large number of torrid super Jupiters (several times the mass of Jupiter, but at temperatures of several 100 degrees). Also hot Neptune-size planets have been found. Many earth-like planets have also been found, one of them having much larger quantities of water than present on earth.

Many of them are super hot. Among the isolated objects, astronomers have also been discovering hundreds of so-called brown dwarfs which are objects intermediate in mass between stars and planets. They are not massive enough to have sufficient heat in their core, to trigger sustained nuclear reactions, to shine as stars, but are too massive to be planets. A celestial object must be at lease 70 times Jupiter’s mass to trigger nuclear reactions, converting hydrogen to helium to shine steadily for billions of years as stars. Instead, brown dwarfs briefly shine as reddish brown objects (from their heat of formation) and then fade.

Hence, they are called brown dwarfs. Some of them (if they are around 15 times Jupiter’s mass) can convert deuterium to helium for some time. Most brown dwarfs are too hot at the surface (well above a 1000 degrees). Surprisingly, using the infrared Spitzer telescope, Luhman and colleagues discovered a brown dwarf whose surface is at a comfortable 30 degrees Celsius. Water can exist as liquid there!

The next coolest brown dwarf is at a close of 100 degrees, i.e., boiling point of water. These objects would be emitting about 50 exawatts of power. Apart from radioactive energy release, deuterium fusion and perhaps a slow shrinking of the object could release this much of energy. In fact, Jupiter’s internal energy source is also not fully understood. Models include gravitational energy of falling helium droplets into the core or a slow shrinking of the object.

In short, we can now have a whole zoo of freely floating planets (or low mass sub-stellar objects like brown dwarfs) with exotic properties, leading lonely lives, independent of the confines and constraints of parent stars. There could be several more of them than ordinary bound planets, vastly increasing the possibilities for exobiology!